Static files should be served respecting the base-url

Let's say I proxy my wiki using nginx to `/wiki/`. The serving of static files and such will break with this. Currently, the documentation appears to insist one should use Apache2 with some HTML rewriting module instead, or simply set up a subdomain for the wiki. This seems suboptimal.

A configuration option called `base-url` already exists. If gitit were to use the base-url as a base for static files and links, no fragile/expensive HTML rewriting would be needed, as gitit itself could just return the canonical URL we'd like to have it served at.